Cerebrospinal fluid (“CSF”) serves several important roles in the human body. The CSF provides buoyancy to the brain, which allows the brain to maintain its density without being impaired by its own weight. Without CSF, the weight of the brain would cut off blood supply in the lower sections of the brain, which could result in the death of neurons in these areas.
CSF protects the brain tissue from injury when jolted or hit. CSF thereby reduces the potential of hemorrhaging, brain damage or death caused by the brain being forced into contact with the skull.
CSF flows throughout the inner ventricular system in the brain and is absorbed back into the bloodstream, rinsing the metabolic waste from the central nervous system through the blood-brain barrier. This process allows for homeostatic regulation of the distribution of neuroendocrine factors, to which slight changes can cause problems or damage to the nervous system.
CSF is produced in the brain at a rate of between about 20 ml/hr and about 30 ml/hr. In a normal human, the CSF is absorbed into the body at a rate that is approximately the same as the rate at which the CSF is generated. This configuration enables the intracranial pressure to remain substantially consistent.
The human cranial compartment is incompressible which causes the volume inside of the cranium to be fixed. Within the cranium are located brain tissue, CSF and blood. These components are in a state of volume equilibrium such that any increase in the volume of one of these components must be compensated by a decrease in the volume of one of the other components.
A principal buffer within the cranium is CSF, the volume of which responds to increases or decreases of the other components within the cranium. Through such a change, it is possible to maintain the intracranial pressure within a normal range. Such a process is typically only effective for changes of volume of less than about 120 ml.
When in a laying down position, the typical intracranial pressure for adults is between about 7 and 15 mmHg. If the intracranial pressure exceeds 25 mmHg, a person may experience headache, vomiting, loss of consciousness, blindness and even death. Treatment is thereby warranted to reduce the intracranial pressure.
One technique that may be used to reduce the intracranial pressure is to provide more space for the brain tissue, CSF and blood by removing a portion of the person's cranium. Because of the potential risks of not having a complete cranium, this technique is generally reserved for only those situations where there are no other alternatives to maintain the intracranial pressure within the desired range.
While the increased intracranial pressure is far more common, it is also necessary to ensure that the intracranial pressure does not drop too low. The symptoms for intracranial hypotension are often similar to the symptoms of intracranial hypertension. As such, many medical experts believe that the symptoms are caused by the change in intracranial pressure as opposed to the pressure itself.
Based upon the preceding comments, it can be appreciated that it is important to prevent the amount of CSF from becoming too large or too low. For example, hydrocephalus is a medical condition that occurs when CSF builds up in the ventricles of the human brain. This build-up causes an abnormal and dangerous increase in intracranial pressure.
The typical procedure for treating hydrocephalus is to insert a drainage catheter into the ventricles of the brains. The drainage catheter enables the excess CSF to be diverted to another region of the human body where the CSF may be absorbed.
Catheters inserted for such a purpose are termed “proximal catheters.” Some proximal catheters have one inlet hole by which CSF enters. Others have inlet holes along their longitudinal axis that vary in number, shape, distribution, and entrance conditions. The drainage section of these catheters is termed the “proximal end” and the end opposite the drainage section is the “distal end.”
The flow rate of the CSF to be drained may be quite low. As a result, the CSF inflow into the various inlet holes of catheters used to treat these conditions is also quite low. It is commonly thought that only 1 or 2 inlet holes are required to permit adequate flow through proximal catheters and that most inlet holes are redundant.
A problem associated with CSF drainage catheters is tissue in-growth into the catheter holes. One potential factor believed to cause the tissue in-growth is the flow of CSF, which draws the choroid plexus tissue into the holes in the catheter.
It has been widely published that proximal catheters have a 30-40% chance of requiring emergency repair in the first year, and an 80% chance of failure after twelve years of implantation. The primary cause of the mechanical failures for these catheters is blockage of the most proximal inlet holes. Blockage is typically caused by CSF debris such as blood clots, cell clusters, brain parenchyma, and choroid plexus and ependymal tissue.
A study of proximal catheters was performed using the analytical tool of computational fluid dynamics (“CFD”). The purpose of the study was to determine the dynamics of inflow into the inlet holes of those catheters. The results of the study demonstrated that about 70% of the inflow into catheters having inlet holes of equal area occurred in the most proximal inlet holes.
FIG. 5 shows the inflow distribution into a typical proximal catheter having sixteen inlet holes of equal cross-sectional area at eight inflow positions. An “inflow position” occurs at any position along the longitudinal axis of the catheter where at least one inlet hole is located. Inlet hole numbers 1 and 2 illustrated in FIG. 5 are located at the most proximal inflow position, i.e., the drainage end of the catheter.
As illustrated in FIG. 5, at low inflow rates fluid inflow into the various inlet holes of drainage catheters is not uniform. This disproportionate inflow causes a disproportionate amount of debris to be deposited within these inlet holes as well as in the catheter passageway at the location of these inlet holes. Because these most proximal inlet holes are located at the drainage end of the catheter, blockage at this point results in drainage failure for the entire catheter.
Numerous designs have been attempted to guard against debris being deposited onto and into drainage catheters. Some attempts have been made to add physical guards to the external surface of drainage catheters.
Other attempts have focused on the valves that are used to regulate the fluid flow out of the catheters. The present invention, however, focuses on the catheter inlet holes themselves and the fluid dynamics that underlie the mechanics behind fluid entry into those inlet holes.
Wolter, U.S. Pat. No. 5,451,215, discloses a suction drain that is described as being intended for use with removing discharges from wounds. The Wolter catheter includes a tubular structure with a distal end and a proximal end.
A cap containing antibacterial material is attached to the distal end to substantially seal the distal end. The cap allows the antibacterial material to be discharged into the region surrounding the distal end to minimize the potential of an infection developing in such region.
Proximate the distal end, a plurality of openings are formed in the tubular structure. The diameter of the openings decreases in the direction of suction. This configuration provides a suction effect that is approximately the same in the region of such openings.
The proximal end is attached to a source of reduced pressure such as a vacuum source. This reduced pressure causes discharge proximate the distal end to be drawn through the openings, into the tubular structure and then towards the proximal end. This negative pressure is typically on the order of several inches of H2O.
Wolter indicates that this configuration causes the antibacterial material to move lengthy distances through the body tissue until the antibacterial material is removed by suction through the openings that extend along opposite sides of the tube.
In wound drainage, it is desired to draw debris present in the wound into the catheter. Through such a process, the debris is removed from the wound, which enhances the rate at which the wound heals.